A logo usage guide is a focused document covering the specific rules for applying a logo correctly — clear space, minimum size, approved color and background combinations, and common misuses to avoid.
It's typically a subset of a full brand style guide, useful as a standalone reference when you only need to hand someone (a print vendor, a partner, a new hire) the logo-specific rules without the entire brand system.
Having this as an accessible, easy-to-share document — rather than assuming people will intuit correct usage — is one of the simplest ways to prevent a logo from being stretched, recolored or crowded incorrectly over time.